PO items becomes 'free'

First of all i wanna thank for your patience to my poor english.

Working with Retail.

Mostly, we are using PO's to make goods movement between different site's attached to different company code's, and there is no any problems. But sometimes we need to create PO for goods movement between two different site's attached to one company code and add some extra conditions in that PO's header. When i'm creating that sort of order system marking items as free by default. It comes with obvious consequences: there is no conditions for free items, all parameters of invoicing unmarked. After i filled my PO with items, i'm unmarking 'free' flags on items manually, go to save after that and here problem appears! While saving PO, system returns 'free' flags on random items (1 or 2 usually from PO containing 10 - 15 items). In a result i have an order with conditions in header that applyed only to 'no free' items, and this is not good because users usually misses this situation.

All i wrote is true for ME21N, but goes ok for ME21.

Asking for help in finding a way to teach ME21N how not to mark items as 'free' by it self. Or how to configurate system in such a way that items will not be 'free' by default on movements between two sites attached to one company code.